Clinical course
Remember that a whoop is rarely seen in young children and often also not seen when older patients present. The many outbreaks over the last 10 years have confirmed that paroxysmal cough with/without apnea in an infant/toddler should raise our index of suspicion. Likewise, older children, adolescents, and adults with persistent cough beyond 2 weeks are potential pertussis cases. Once the diagnosis is made, treatment is not expected to have a major impact on the clinical course, in part because the diagnosis is usually delayed (more than 10 days into symptoms). This delay allows more injury to the respiratory mucosa and cilia so that healing can require 6-12 weeks after bacterial replication ceases. This prolonged healing process is what is mostly responsible for the syndrome known as the “100-day cough.” So the clinical course of pertussis has not changed in the last 10 years. However, there have been changes in the commonly used diagnostic approach.
Pertussis diagnosis and contagion
During the last 5 years, polymerase chain reaction (PCR) testing has become the preferred technology to detect pertussis. This is due to the sensitivity and quick turnaround time of the assay. The gold standard for pertussis diagnosis remains culture, but it is expensive, cumbersome, and slow (up to a week to provide results). An ongoing debate arose about how long PCR testing remains positive after the onset of symptoms or treatment. This was not the problem when culture was the diagnostic tool of choice. Data from the 1970s and 1980s indicated that cultures were rarely positive after the third week of symptoms even without treatment. Furthermore, macrolides eliminated both contagion and positive culture results of infected patients after 5 days of treatment.
So now that we use PCR most often for diagnosis, what is the outer limit of positivity? A recent prospective cohort study from Salt Lake City suggests that PCR may detect pertussis DNA way beyond 3 weeks after symptom onset (J. Ped. Infect. Dis. 2014;3:347-9). Among patients hospitalized with laboratory-confirmed Bordetella pertussis infection, half had persistently positive pertussis PCR testing more than 50 days after symptom onset, despite antibiotic treatment and clinical improvement. The median (range) for the last day for a positive test after symptom onset was 58 days (4-172 days).
This raises the question as to whether there are viable pertussis organisms in the respiratory tract beyond the traditional 3 weeks defined by culture data. It is likely that DNA persists in the thick mucus of the respiratory tract way beyond viability of the last pertussis organisms. Put another way, PCR likely detects bacterial corpses or components way beyond the time that the patient is contagious. Unfortunately, current PCR data do not tell us how long patients remain contagious with the current strains of pertussis as infecting agents. Some institutions appear to be extending the isolation time for patients treated for pertussis beyond the traditional 5 days post initiation of effective treatment. Until more data are available, we are somewhat in the dark. But I would take comfort in the fact that it is unlikely the “new” data will be much different from those derived from the traditional studies that use culture to define infectivity. The American Academy of Pediatrics Committee on Infectious Diseases Red Book appears to agree.
For hospitalized pertussis patients, the AAP Committee on Infectious Diseases Red Book recommends standard and droplet precautions for 5 days after starting effective therapy, or 3 weeks after cough onset if appropriate antimicrobial therapy has not been given.
In addition, the CDC states: “PCR has optimal sensitivity during the first 3 weeks of cough when bacterial DNA is still present in the nasopharynx. After the fourth week of cough, the amount of bacterial DNA rapidly diminishes, which increases the risk of obtaining falsely negative results.” Later in the same document, the CDC says: “PCR testing following antibiotic therapy also can result in falsely negative findings. The exact duration of positivity following antibiotic use is not well understood, but PCR testing after 5 days of antibiotic use is unlikely to be of benefit and is generally not recommended.”
So what do we know? Not all PCR assays use the same primers, so some variance from the usual experience of up to 4 weeks of positive PCR results may be due to differences in the assays. But this raises concern that the PCR that you order may be positive at times when the patient is no longer contagious.
Pertussis treatment
If strains of pertussis have changed their pertactin antigen, are they changing their antibiotic susceptibility patterns? While there have been reports of macrolide resistance in a few pertussis strains, these still remain rare. The most recent comprehensive review of treatment efficacy was a Cochrane review performed in 2005 and published in 2007 (Cochrane Database Syst. Rev. 2007;3:CD004404). They evaluated 10 trials from 1969 to 2004 in which microbiologic eradication was defined by negative results from repeat pertussis culture. While meta-analysis of microbiologic eradication was not possible because of differences in antibiotic use, the investigators did conclude that antibiotic treatment “is effective in eliminating B. pertussis from patients with the disease to render them noninfectious, but does not alter the subsequent clinical course of the illness.”